1. Field of the Invention
The invention relates to a rate control method with region of interesting support for increasing the image quality of the region of interesting.
2. Description of the Related Art
Compressing methods are typically used to compress the pictures in order to reduce the data amounts, for example coding pictures by MPEG. The basic unit of coding within a picture is the macroblock. If the sampling is in 4:2:0 format, there are six blocks within a macroblock: four Y blocks, one Cb block and one Cr block. Each block is first DCT (discrete cosine transform) transformed, and then the DCT coefficients have to be quantized into integer. A zigzag scan or other alternate scanning method is utilized to arrange the two-dimensional quantized coefficients into one-dimensional. Finally, a variable-length coding (VLC) is employed to do entropy coding.
The quantization step is where the compression happened. Generally, the quantization of a DCT coefficient F[v][u] can be represented as:QF[v][u]=16*F[v][u]/(Q*W[v][u])  (1)
Where the v, u are the indexes for the two dimensional matrix, ranged from 0 to 7. The Q is the quantizer scale for the blocks within a macroblock, which can be varied for each macroblock. The W[v][u] is a quantization matrix defined for the whole pictures, which is used to assign a weighting factor for different DCT coefficients. FIG. 1 is a default quantization matrix for intra blocks defined in MPEG-2. Referring to FIG. 1, the W[v][u] is getting larger when the v and u indexes increase. This causes that the quantized coefficients with higher frequency more easily becomes zero. This is based on the study of human visual system that human is more sensitive to lower frequency signal and less sensitive to higher frequency.
Generally, an encoder treats an image as a single object to encode. The visual quality of the whole image would be uniform. However, in some applications, we would like a dedicate region of the image to be coded with a better quality than other region. The surveillance system is a good example. Typically, there are multiple cameras connected to a single recording system. The images came form these cameras are shrunk and merged into a single image. Thus, a single recording system can be used to record the images of multiple cameras. However, if we think that some cameras should have higher priority than other cameras, the traditional recording system can't satisfy this requirement. Another example is the encoding of video sequence with captions blending onto the images. Because the caption is frozen over some images but the background is moving, the ringing artifact around the caption becomes obviously. If this area is given more bit rate, the overall visual quality of the image could become better. The region on an image that we want to get a better quality than other region is called the region of interesting (ROI).
However, because the target bit rate for a video sequence is constrained, and thus the bit budget for each picture is a limited resource, how to allocate the bit budget for each region according to its priority to achieve the effect of ROI that user required is not a trivial problem.